Method of copying utilizing an infrared-absorptive image formed by electrostatic attraction



Feb. 13, 1968 R. OWEN 3,368,892

METHOD OF COPYING UTLZING AN INFRARED-ABSORPTIVE IMAGE FORMED BY ELECTROSTATIC ATTRACTION Original Filed Aug, l0, 1959 United States Patent Office 3,368,892 Patented Feb. 13, 1968 3,368,892 METHOD OF COPYING UTILIZING AN INFRA- RED-ABSDRPTIVE IMA-'GE FORMED BY ELEC- TRSTATIC ATTRACTION Richard Owen, Brooklyn Center, Minn, assignor to Minnesota Mining and Manufacturing Company, St. Paul, Minn., a corporation of Delaware @riginal application Aug. 10, 1959, Ser. No. 832,793, now Patent No. 3,328,167, dated June 27, 1967. Divided and this application June 12, 1967, Ser. No. 645,406

3 Claims. (Cl. 96-1) ABSTRACT F THE DISCLOSURE Exposure of a copy-sheet to a light-image forms a latent image which is developed to a differentially infrared-absorptive image and then irradiated to cause localized heating and image formation in a heat-sensitive layer of the sheet.

Specification This application isa division of copending application Ser. No. 832,793, filed Aug. 10, 1959, and now U.S. Patent No. 3,328,167.

This invention relates to the formation of permanent visible records or reproductions of light-images.

In one aspect, the present invention makes possible the formation of permanent stable reproductions -of lightimages while avoiding any necessity for chemical stabilization or fixing, or other treatment with aqueous solutions, vapors, or -other external chemical agencies. The invention also makes possible the intensification of indistinctly visible reproductions of light-images obtained by more conventional methods and which may involve initial chemical or other image-development. Thus, the invention permits the effective utilization of light-sensitive mechanisms which of themselves do not directly provide visible copies, or copies of adequate intensity or of an effective degree of stability. The invention also provides means for directly producing visible reproductions of low intensity lightimages on heat-sensitive copy-papers, and hence makes possible such procedures as the making of full-scale or enlarged reproductions of original graphic subject-matter from reduced photographic negative copies, or microfilm copies, of such subject-matter. The invention furthermore permits the copying of al1 visible subject-matter, including the making of portraits or the like as well as the copying of colored ink drawings, signatures, paintings, etc., on heat-sensitive copy-paper.

The invention will now be described in terms of illustrative but non-limitative specific embodiments.

Example I y A transparent, visibly heat-sensitive copy-paper is first prepared by coating a transparent polyester lm backing (Mylar lrn) with a mixture of ferric stearate and t-butyl catechol in a solution of ethyl cellulose binder, and removing the solvent by evaporation at room temperature. The resulting sheet has a faint buff color, changing rapidly to blue-black when heated, e.g. to about 100 C. It is suitable for making reproductions of typewritten corresnondence or'other graphic originals by methods disclosed in Miller et al. U.S. Patents Nos. 2,663,654-7. which also describe various additional typical heat-sensitive copy-sheet structures and formulations having utility inthe present invention.

Over the heat-sensitive layer is next applied a thin coating of a solution of equal parts by weight of silver nitrate and polyvinylpyrrolidone binder in a solvent mixture of two parts ofrwater and nine parts of methyl alcohol. The solution is preferably prepared, coated and dried under darkroom conditions or at least under subdued light.

The resulting sheet is Contact printed by exposure -of the coated surface for six minutes to ultraviolet radiation from a BH-6 lamp at a distance of six inches and through a photographic negative held in contact with the sensitized surface. Close inspection of the thus exposed sheet shows a very faint brownish image corresponding to the transparent areas of the photographic negative.

The sheet is then briefly irradiated with high intensity light rich in infrared rays. Apparatus for carrying out this step of the process is described in the Miller et al. patents hereinbefore referred to; and see also Miller U.S. Patents Nos. 2,740,895-6. A dense blue-black image on a buff background is formed, corresponding to the initial faint brownish image, and fully visible through the transparent film backing. The copy as thus viewed is stable on long aging even though the back surface gradually darkens te a uniform gray-brown appearance or is otherwise altered.

Example 2 A heat-sensitive copy-sheet consisting of a thin heattransparentizable opaque water-resistant coating of hydrogenated fatty oil wax particles and cellulosic binder on a thin transparent paper support member previously coated with a thin layer of titanium dioxide pigment, an infrared-transmitting blue like pigment, and polymeric binder, all as more fully described in Clark et al. U.S. Patent No. 2,710,263, is supplied on the uncoated surface of the support with a Very thin layer of silver halidegelatin photographic emulsion, applied and dried under darkroom conditions. The sheet is briefly exposed to an enlarged light-image projected from a microfilm transparency, producing a latent image, and a corresponding visibile image is developed by treatment of the silver halide layer with a conventional photographic developer solution. A faint black image is obtained, the darker areas corresponding to the light-struck areas. The dried sheet is then exposed to brief intense irradiation as in Example l, resulting in the selective transparentizing of those portion-s of the waxy coating overlying the developed visible image. A high contrast reproduction of the original light image is thus obtained, having deep blue image areas against a white :background as viewed from the Waxcoated side. Further exposure to light of the unstabilized silver halide layer causes a general darkening of said layer, but without impairing the legibility of the image as viewed from the heat-sensitive side. This darkening may be avoided by fixing and washing if desired.

The procedure just described permits the taking of photographs `or the reproduction of printed or other graphic originals with minimum exposure to light, but requires the additional step of chemical development of the latent image. Similar sheets may be used in the procedure described in connection with Example 1, but with increased exposure to directly provide a temporarily useful intermediate reproduction of the type obtained with photographic print-out papers.

Example 3 A visibly heat-sensitive copy-paper of the type described in connection with Example 1 is coated with a thin layer of an iron ammonium citrate-potassium ferricyanide solution as employed in making blueprint paper.

.Mixing coating, and drying are carried out under darkirradiated with infrared to develop a corresponding permanent image in the visibly heat-sensitive layer. Alternatively, the intermediate print may be irradiated without being stabilized, in which case the entire light-sensitive layer eventually darkens while the heat-sensitive layer retains the stabilized image.

Example 4 The light-sensitive blueprinting composition of Example 3 is similarly applied to a transparent supporting web which carries on the reverse surface a heat-transparentizable opaque coating of a mixture of fusible wax particles and non-fusing binder as described in connection with Example 2. The intermediate image produced on exposure of the light-sensitive layer to a light-image is reproduced in the heat-sensitive layer, by exposure to radiation high in infrared, in the form of transparent image areas in a white opaque background. The blue color of the intermediate image is visible through these transparent areas, producing a blue-and-white stable print.

Example 5 A transparent supporting web is provided on one major surface with an infrared-absorptive visibly heat-sensitive layer prepared by incorporating titanium dioxide pigment in a ferrie stearate-gallic acid-ethyl cellulose heat-sensitive composition. A white or light buff opaque coating is thus obtained which is convertible to a dense blue-black by contact with a heated article or by momentary exposure to appropriate high intensity illumination. The supporting web is provided on the opposing major surface with a slightly acidic light-sensitive layer containing a diazonium salt and a coupling agent, as used in the preparation of diazotype printing plates and the like, producing a faint yellow color.

The diazo-coated surface is first exposed to a lightimage by exposure through a photographic negative transparency, and is treated with ammonia to develop an intense colored image on a white or slightly yellowish background. The resulting print is also a negative; i.e. the sensitive layer develops the intense color only in those areas not exposed to light.

The sheet is then exposed from the same surface to high intensity ash irradiation such as is produced by a 500 watt-second discharge between terminal electrodes in a lO-inch quartz tube lled with xenon under reduced pressure, the flash having a duration of about 1000 microseconds with peak intensity at about 100 microseconds. Darkening of the heat-sensitive layer occurs in areas corresponding to the background areas of the previous negative print, thus producing a positive print having blueblack image areas on a white or light buff background.

Any diazotype light-sensitive layer may be used which produces image areas capable of absorbing the radiations applied. In the specific example, the image areas are converted to an intense yellow. The background areas remain a faint yellow or are bleached to an off-white.

The intensity of the ash irradiation is sufiicient to cause blistering or other degradation of a photographic transparency. The procedure described permits the production of a positive print from a negative original on a heatsensitive copy-paper without danger of damage to the original through overheating during exposure.

Ex'am ple 6 Thin transparent paper (map overlay tracing paper) is first coated with a smooth uniform layer of a fiuid dispersion of 125 parts by weight of ferrie stearate and 35 parts of alcohol-insoluble precipitate of hexamethylenetetramine and pyrogallic acid in a solution of parts of polyvinyl butyral in suicient alcohol to provide a coatable mixture. The coating is dried at room temperature, leaving a residual dried layer of approximately 0.7 lb. per sq. yd. A second coating of a suspension of about 12 parts of zinc oxide pigment in a solution of about 71/2 parts of ethyl cellulose in acetone is next applied and dried, producing a thin visibly opaque coating. The sheet is stored for several hours under dark Conditions and is then colzted over the zinc oxide layer with a thin layer of silver nitrate solution, e.g. by lightly rubbing with a cotton swab moistened with the solution, the operation being performed under subdued light. After drying, the coated sheet is first exposed to a light-image, causing darkening of the light-struck silver-coated areas. 1t is then momentarily intensely irradiated with infrared on the imaged surface, producing a corresponding but much more distinct image in the heat-sensitive layer which is visible through the transparent paper support layer. Further exposure to light causes uniform darkening of the silvered surface without affecting the visible image in the heatsensitive layer.

Example 7 In this example the zinc oxide-ethyl cellulose layer of Example 6 is applied directly to the thin paper backing. After a suitable aging period under dark conditions, silver nitrate solution is swabbed over the coating and the sheet is dried, again in absence of light. The resulting sheet darkens on exposure to light. Prior to such exposure it is provided on the uncoated surface with a thin layer of a heat-sensitive composition of equal molar proportions of silver behenate, behenic acid, and protocatechuic acid, together with resinous binder material. The sheetis first exposed on the first-coated side to a light-image, producing a visible but slight darkening at the light-struck areas. It is then briefly exposed on the same surface to high intensity infrared radiation. The darkened image areas differentially absorb the radiation and the resulting heatpattern causes a corresponding visible change in the heatsensitive layer. During irradiation, the heat-sensitive surface is preferably insulated from contact with heat-absorptive surfaces. Viewed from the heat-sensitive surface, the copy is a black-on-white mirror image of the original white-on-black light-image.

Zinc oxide is a preferred example of a particulate light-sensitive electron donor material. Other examples include photochromic complex compounds such as [Co(C2H.,-.N2H4)3]2(S2O8)3 and photoluminescent pigment materials such as silver-activated zinc sulfide. Such materials are known to provide free electrons under the action of ultraviolet light or equivalent actinic radiation. Likewise, silver nitrate is a preferred example of electron acceptor material which undergoes a visible change in contact with the thus irradiated zinc oxide; gold chloride and chloroplatinic acid act similarly under the same conditions. The color change occurring in the silver nitratezinc oxide layer on exposure to a light-image is sufficient to provide a degree of contrast adequate for many purposes, but the image is not permanent under subsequent illumination. On the other hand, the image obtained in the heat-sensitive layer is stable to light and provides an image of improved contrast.

Example 8 Thin transparent paper is first provided with a heatsensitive layer as described in connection with Example 1, and is then further coated with a thin continuous layer of four parts of strongly photoconductive zinc oxide in one part of the resinous copolymer of equal parts of styrene and isobutylene. The volatile ketone vehicle is removed and the sheet stored under dark conditions.

For use, the sheet is placed against a grounded fiat metal surface with the coated surface uppermost and is electrostatically charged from a conductor adjacent thereto at a potential of about 2,000 volts. The sheet is then exposed to a light-image, permitting dissipation of the electrostatic charge at the light-struck areas. The resulting image pattern is made visible by development with colored electrostatically charged powder. The powder adheres to the charged areas but not to the discharged arcas, producing a copy of the original light-image. The copysheet is then suitably isolated from heat-conductive surfaces and the imaged area is briefly intensely irradiated. The radiation is preferentially absorbed at the powdered areas and the resulting heat-pattern produces a corresponding visible image pattern in the heat-sensitive layer which is seen as Ia mirror image through the transparent paper carrier web.

Where a fusible colored powder is used in developing the electrostatic image, or a fusible binder is used in the photosensitive layer, the powder is found to be permanently bonded to the surface of the sheet. Such action is not required, however, and the powder may as effectively be completely removed, as by brushing, `'once the image has been made permanent in the heat-sensitive layer.

Various other light-sensitive materials =may be substituted for the exemplary materials hereinbefore suggested. They may, for example, be self-developing, as with the blueprint paper composition `of Example 3 or the silver nitrate of Example 1 or the Zinc oxide-silver nitrate combinations of Examples 6 or 7; or they may require chemical development of a latent image as with the silver halide layer of Example 2 or the diazotype coating of Example 5; or lphysical development may be indicated, as in the case of Example 8. Various other heat-sensitive layers may likewise replace the chemically reactive colorproducing layers of Examples 1, 3, or .5-8, or the physically transparentizable layers of Examples 2 and 4. Furthermore the copy-forming heat-pattern may be produced in the ylight-image areas, or, yas in Example 5, in the heat-sensitive layer itself. Various other combinations and modifications will occur on consideration of the disclosures hereof. yIn all cases, there is formed at the lightsensitive layer a pattern of image and background areas which `differ in visible appearance and in ability to absorb light, thus permitting the subsequent development of a corresponding heat-pattern by momentary intense irradiation, and resulting in the production of a corresponding visible print pattern in the heat-sensitive layer. Although the radiation :may be primarily absorbed and the heating effect produced either in the darkened areas lof the lightsensitive layer or in the unmasked yareas of the heatsensitive layer, it Will be seen that in lall cases the visibly different image and background areas of the converted light-sensitive layer will be significantly dissimilar in their ability to absorb the radiations employed in producing the heat-pattern with which the final print is developed.

A typical light-and-heat-sensitive copy-sheet as hereinabove described is illustrated in Ithe accompanying drawing, showing a copy-sheet compri-sing a supporting web 10, a visibly heat-sensitive coating 11, and a light-sen-sitive coating 12. Various combinations of these components are contemplated; thus the two sensitive coatings may be on opposite sides of an extremely thin support web, as indicated in Examples 2 and 7, or preferably on the same side as in the remaining examples, and either coating outermost. Where the heat-sensitive coating is between the light-sensitive coating and the support, the Ilatter will normally be transparent; and this structure is generally preferred, since the printed heat-sensitive coating is thus protected by the -transparent support, and the latter, particularly where a thin paper is employed, provides an effective marking surface which will accept subsequent pencil or -other markings. yThe support web may be omitted, its function then being provided by the filmforming .binder components of either or both of the sensitive layers. On the other hand, additional layers, e.g. transparent protective surface coatings, may be added if desired. In some instances such layers assist in providing intense print-images in `the heat-sensitive layer by helping to prevent loss of heat from lthe composite sheet during irradiation of the intermediate image. Alternatively, separate temporary heat barriers may 'be provided for this purpose where desired.

The image-intensifying action of the copy-sheets of this invention make possible Ithe effective use of ordinary less desirable light-sensitive compositions. Thus, light-sensitive coatings which by themselves yield images of loW visible intensity, or which are unstable when exposed to light or other conditions of use, or which require prolonged or complicated treatment rto stabilize, may be employed in these constructions vwith formation of fully effective permanent reproductions of light-images.

What is claimed is as follows:

1. Method of making a permanent record of a lightimage comprising: exposing to said light-image a copysheet having lan electrostatically sensitized photoconductive layer and -a light-resistant heat-sensitive thermographic layer, to provide on said photoconductive layer a latent image corresponding to said light-image; developing said latent image by application of infrared-absorptive electrostatic developing powder, with removal of excess powder; and briey exposing said sheet to intense infrared radiation to cause heating at the developed image areas vand formation of a corresponding visible thermographic image in said thermographic layer.

2. The method of claim 1 wherein Ithe powder image is removed after formation of the thermographic image.

3. The method of claim 1 wherein the powder image is permanently fused to the copy-sheet during said heating.

References Cited UNITED STATES PATENTS 2,692,178 10/1954 Grandadam 346-74 2,798,960 7/ 1957 Moncriei-Yeates 250-65 2,939,787 6/1960 `Giaimo 96-1 I. TRAVIS BROWN, Acting Primary Examiner. C. E. VAN HORN, Assistant Examiner, 

